Many systems have been proposed to provide operator's inputs in a cathode ray tube display system by providing, at the cathode ray tube screen face, an arrangement sensitive to an operator positioning an object or his finger on the face.
Probably the most well known of such devices is the light pen. This is a light sensitive device which, when placed on a cathode ray tube face, detects the light generated by the cathode ray tube beam as it sweeps past the pen. The position of the pen on the screen can be calculated by reference to the timing of the cathode ray tube beam sweeping. The main disadvantages of the light pen are that it has to be coupled to the display system by a flying lead connector which can prove awkward in use and that it has to be robustly constructed to prevent damage due to dropping.
In order to overcome the disadvantages of the light pen, there have been a number of proposals to provde interactive cathode ray tube inputs by contacting the cathode ray tube face. An article entitled `Touch Sensitive CRT Screens Join Computers and Nonusers` in Electronic Design, Volume 29, No. 21, Oct. 15, 1981 at pages 61, 62 and 64 summarizes such systems. There may be divided into two general groups, passive devices and active sensory devices.
The passive devices comprise matrices of resistive capacitive elements formed on membranes which are fixed to the face of a cathode ray tube, or circuit elements fixed directly to the face. With the resistive devices, touching a resistive membrane signals the position of the touch by contact between normally spaced resistive surfaces at the touch area via a voltage divider effect. With the capacitive devices, touching one of a pattern of pads on the cathode ray tube face adds the human body's capacitance to that pad, and a circuit detects the change and signals the position of the touched pad. U.S. Pat. No. 3,482,241 shows one form the circuitry used to sense capacitive touch pads. One problem with these passive devices is that they obscure or reduce the brilliance of areas of the cathode ray tube screen.
To avoid this problem, the active devices were developed. As indicated in the above mentioned Electronic Design article, these devices flood the screen with acoustic or light signals. With the acoustic system, piezoelectric transducers positioned adjacent the screen produced acoustic surface waves across the screen. These are reflected by any object touching the screen face and the reflected signals are sensed by the transducers to provide an indication of the position of the object. The optic systems use an array of L.E.D.'s at one side of the screen which generate light beams which are sensed by photo diodes at the opposite side of the screen. In these systems, breaking of the light beams by touching the cathode ray tube face is sensed to provide an indication of the touch position. These active systems are relatively expensive and are subject to ambient signal interference, though steps can be taken to minimize such interference.
U.S. Pat. No. 4,281,323 shows a display touch sensing system which uses ambient electrical noise generated by the display device. In that system, conductive strips are arranged in rows and columns across the face of a display device. In operation of the display device, electrical noise is generated constantly in the strips. When an operator places his finger at the conjunction of a row and a line strip, the amplitude of the noise on these strips changes. This change is sensed to provide an indication of the position of the touch. This system, as it employs lines and pads on the screen face, has the same disadvantage as that of the passive devices described above.